There is a lot of talk of semantic HTML in the developer world lately, but how does this weigh up against something like the 960 grid system or twitter boot strapper..

If i wanted to quickly deploy a web app or site and decided to throw something together with either of the above my code would be full of div classes like containers, rows, spans, offsets, pulls etc..

How damaging is this for the site? does it effect search ranking? Surely it's the content that they care about.. is semantic code only beneficial for the developer? or developers working in a team?

I guess this has more of an impact when using html5, i guess using article, aside, header, nav, footer correctly will have a positive effect with search engines?

If i was to use older methods such as div class="header", "nav", "sidebar", the search engine wont know the difference if i was to use div class="span16", "row", etc.